Venus' missing water is a planetary mystery

Venus, Earth similar in size, likely began with similar amounts of water

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Venus, like Earth and Mars, is immersed in a flow of charged gas from the sun. Since Venus (and Mars) doesn't have a magnetic field, gases in the upper atmosphere become charged and interact with the solar wind. Scientists think the solar wind gives the charged particles enough energy to escape and that's why Venus is losing its atmosphere.

The water in Venus' atmosphere is gone with the wind, new detections suggest.

This absence is strange, because astronomers think Venus and Earth likely began with similar amounts of water since they are about the same size and formed at the same time (some 4.5 billion years ago). Yet today, Earth's atmosphere and oceans contain 100,000 times the total amount of water on Venus.

Due to a greenhouse effect on Venus, temperatures at the surface can soar to about 870 degrees Fahrenheit (465 degrees Celsius). And so the water from the planet's surface immediately boils off. But its atmosphere is also relatively dry, and the question has been: where did that initial atmospheric water go?

Magda Delva of the Austrian Academy of Sciences and her colleagues set out to understand this water loss with data collected by instruments onboard the European Space Agency's Venus Express spacecraft.

They found evidence for the loss of hydrogen from the atmosphere on Venus' day side, or the side facing the sun. The finding suggests the solar wind, a stream of charged particles from the sun, could be responsible for stripping away the hydrogen atoms. The hydrogen may have been part of water molecules (H2O).

The solar wind carries electrical and magnetic fields throughout the solar system and "blows" past the planets. Earth's magnetic field protects us from the charged stream. But Venus doesn't generate a magnetic field. There, the solar wind strikes the planet's upper atmosphere and carries particles off into space.

This day-side process is responsible for about 20 percent of the hydrogen lost from Venus' atmosphere, Delva told SPACE.com, with the other 80 percent lost on the planet's night side.

Last year, Venus Express data revealed a loss of hydrogen and oxygen on Venus' night side, with twice as many hydrogen atoms as oxygen atoms escaping from the atmosphere. Since water is made of two hydrogen atoms and one oxygen atom, the detection suggested water was being broken up in the atmosphere.

On the day side, however, Delva and her colleagues have yet to detect any oxygen atoms, which would help seal the deal for a watery source. "I keep looking at the magnetometer data but so far I can't see the signature of oxygen escaping on the day side," Delva said.

Compared with the night side, the hydrogen atoms stripped from the day side exist much higher in the atmosphere, at about 6,200 miles to 31,000 miles (10,000 km to 50,000 km) above the surface of Venus. And there are more of them.

"These results show that there could be at least twice as much hydrogen in the upper atmosphere of Venus than we thought," Delva said.

The research will be detailed in the journal Geophysical Research Letters.